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Step 8

3 min read

Primary Key

Learn what Primary Keys are and why every table needs one.

What is a Primary Key?

A Primary Key is a unique ID for each row. Like your student ID or passport number.

Simple rule: No two rows can have the same Primary Key.

How Primary Key Works

Basic Syntax

CREATE TABLE students (
  student_id INTEGER PRIMARY KEY,
  name VARCHAR(100),
  age INTEGER
);

Example 1: Auto-Generated IDs

Let database create IDs automatically:

CREATE TABLE students (
  student_id SERIAL PRIMARY KEY,
  name VARCHAR(100)
);

Example 2: Insert with Primary Key

INSERT INTO students (name) VALUES ('John');  -- Gets ID 1
INSERT INTO students (name) VALUES ('Mary');  -- Gets ID 2

Example 3: Find by Primary Key

SELECT * FROM students WHERE student_id = 1;

Primary Key Rules

Must be unique - No duplicates
Cannot be NULL - Must have value
One per table - Only one Primary Key
Never changes - Don't modify it

Try It Below

Use the playground to practice:

SELECT * FROM students WHERE student_id = 1;
SELECT student_id, name FROM students;

What Comes Next

Next: Learn Foreign Keys to connect tables together.

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